ACARS Decoder App For The Macintosh

To decode ACARS transmissions, you need a VHF radio (such as a regular "police scanner"). You should use an audio cable to connect the radio's audio output (line level or headphone jack) to your computer's microphone input, for best reception.

ACARS is the acronym for Aircraft Communications Addressing and Reporting System. It is a data transmission between airplanes and ground stations. The purpose is to handle many types of traffic that would normally be done by voice in the past. ACARS transmissions are on VHF.

Frequencies

The following frequencies are commonly used:

131.550 Primary Channel worldwide
130.025 Secondary channel for USA and Canada
129.125 Additional channel for USA & Canada
130.450 Additional channel for USA & Canada
131.125 Additional channel for USA
136.700 Additional channel for USA
136.800 Additional channel for USA
131.725 Primary channel in Europe
131.525 European secondary
131.475 Air Canada company channel
131.450 Primary channel for Japan
136.900 European secondary
136.925 ARINC European Channel
136.850 SITA Canadian Frequency

As with all other VHF-Aeronautical transmissions, AM mode is used. Most scanners automatically change to AM mode when tuning the 108-136 MHz aircraft band, but you may want to verify your scanner does this. Make sure to have the squelch turned off!

The transmissions sound like data bursts ranging from about 0.2 second to 1 second in length. Due to the high altitude of aircraft, transmissions from aircraft may be monitored for several hundred miles. Those living close to airports may also hear the ground transmissions sent to aircraft.

Tuning in ACARS transmissions is fairly easy - just select ACARS mode and adjust the radio's volume for proper decoding. Please note - it is very important to have the squelch of your radio completely off, that is, so you always hear the background static. The squelch action of any radio is too slow, and you'll miss the packet!

You will find that some transmissions are garbled or not decoded. Good reception quality and strong signals are required. You'll see many packets are repeated, this is because the ground station didn't get the message, so the aircraft had to re-send it. Since the nicely equipped ground stations don't copy 100%, don't be surprised if your setup doesn't either! I find that under good conditions, slightly less than half of the packets decode properly.

Packet Display

The app displays the transmitted packet, and parses out some of the information, such as the plane's wing number, flight number, etc. It does not attempt to parse out other specific information in the text portion of the packet, which is displayed on the second line. A timestamp is also displayed on the first line.

Here is an example of a received ACARS transmission:

.N655UA  Q0 0   [23 Nov 2014   17:59:23][UAL Boeing 767 25393]
S54A UA0950

N55UA is the aircraft registration, note the leading period(s), to force it to 7 characters.

Q0 is the message type

0 is the downlink block identifier

[23 Nov 2014 17:59:23] is the date and time, added by the app

S54A is the message sequence, minutes and seconds past the hour

UA0950 is the carrier and flight number (United flight 950)

There was no text sent in this message, if there was, it would follow the flight number.

[UAL Boeing 767 25393] This is the type of aircraft, it was automatically pulled from the AIRCRAFT.CSV file, described below.

Suppress Parity Errors can be checked to not display packets that are garbled and have characters with parity errors (incorrectly received bits).

Format can be checked to change to an alternate display format of ACARS packets.

A database of many plane tail numbers is included with the app. If a plane tail number is identified from this database, the plane type will be displayed with the other information.

Many planes are not in this database. To handle additional planes, you may create a file called ACARS.LST, which is a listing of plane registration numbers, along with text that you may enter. When a packet is decoded, the plane registration number is compared to the registration numbers of the planes in the ACARS.LST file. If a match is found, the text from that line of the file is displayed after the timestamp. You may use this file to display text or comments about each aircraft.

To load your ACARS.LST file into the app, select "Load plane data file" from the File menu. This must be a plain text file, not a file in a proprietary word processor format.

An example of the file format follows below:

.N814US This is plane number 1
.N320US This is plane number 2
.NIM5AA This is plane number 3
.N781NC This is plane number 4
.N609AA This is plane number 5

Note that the first 7 characters of each line are the plane registration number, followed by a space. The rest of the line contains the text to be displayed. Leading periods are required to force the registration number to be seven characters long.

Email your comments and questions to support@blackcatsystems.com

Last modified January 25, 2015